Direct digital frequency synthesis (DDFS) is a technique that is useful in systems to generate programmable frequency sinusoidal signals. In a DDFS system, a digital signal represents the desired output frequency as a number of sample points that are located along the curve of a sinusoidal waveform or sine wave. Each sample point is located at an instant of time associated with the sine wave. The rate at which the values cycle for the DDFS output determines the frequency of the signal.
A variety of DDFS techniques are describes in various U.S. Patents including U.S. Pat. No. 5,272,454 to Ikai et. al., U.S. Pat. No. 5,399,998 to Hino, U.S. Pat. No. 6,295,362 to Zhang, and U.S. Pat. No. 7,224,302 to Dornbusch.
In U.S. Pat. No. 5,272,454 a digital FM modulator is described that uses an analog-to-digital converter (ADC) to convert an input audio signal to a digital signal that is then gain scaled with a multiplier. Since the output of their multiplier results in a change in the desired center frequency, an adder is used in conjunction with a read-only memory (ROM) to correct for changes in the center frequency of the carrier. A direct digital synthesizer (DDS) is then used to integrate the corrected signal with a sampling period provided by a reference oscillator signal.
In U.S. Pat. No. 5,399,998 an ADC is again used in a digital FM modulator to convert an audio signal into a digital representation. The digital representation is combined with a center frequency by an adder, and passed to a DDS that again integrates the signal with a sampling period provided by a reference oscillator signal. A counter circuit measures the average frequency of the FM wave from the output of the DDS, which is then processed by an “operation means” to adjust the resultant center frequency for a desired center frequency.
In U.S. Pat. No. 6,295,362 a signal generator includes a digital signal processor (DSP) that processes left and right signal channels and interpolates the signals to create a composite baseband signal. The composite baseband signal is used by a numerically controlled oscillator to create an FM modulated carrier.
In U.S. Pat. No. 7,224,302 a modulator includes an ADC that generates digital signals in response to a stereo input signal, an integrator that processes the digital signals, and a DDFS generates a modulated output signal in response to the integrated digital signals.
A DDFS can be advantageously used in a radio frequency (RF) application. In accordance with Nyquist's theorem, the DDFS must be clocked at a rate at least twice as high as the frequency of the signal represented. Since the DDFS provides a digital representation of the output clock signal, rather than the output clock signal itself, there is no mechanism for local oscillator signals to leak or radiate into other circuits, causing unwanted locking or spurs.
The present disclosure has reviewed the above and other conventional DDFS systems in contemplation of the presently described invention. Various improvements are realized over the conventional DDFS systems as will be described.